1. Field of Invention
The present invention relates to method and apparatus for retrieving the information recorded on a rewritable magneto.sub.-- optical media, more particularly, to the method and apparatus for retrieving the information recorded on the rewritable magneto.sub.-- optical media having a function in which the clock signal to be used as a sampling signal is compensated in response to the phase difference between the clock signal and the regenerated electrical signal from the optical disk.
2. Discussion of the Related Art
A rewritable magneto.sub.-- optical media has been in practical use as an information recording media having a high density rewritable capability. In particular, the rewritable magneto.sub.-- optical media having a recording layer made of the amorphous alloy of rare-earth and transition metals have a remarkable characteristic.
The principle of the rewritable magneto.sub.-- optical media is briefly explained by taking an example. By focusing a laser beam on the surface of a rewritable magneto.sub.-- optical media as a spot whose diameter is as short as the wavelength of the light, the temperature of the spot on the recording layer is raised up to 150.degree. C.-200.degree. C. If the temperature of the recording media heated by the laser beam goes up to Curie temperature (Tc), the magnetization of the spot is disappeared. At this time, if constant magnetic bias field is applied in one direction, magnetic mark (it is called a pit) is recorded on the recording layer by the magnetic inversion occurring when the heated area returns to the room temperature.
Referring to FIGS. 1 and 2, the process for recording the information on the rewritable magneto-optical media is explained. FIG. 1 is a diagram showing a conventional type of recording apparatus, and FIG. 2 is a timing diagram to explain the operation of the apparatus in FIG. 1. Based on the information initially preformatted on the optical disk, a channel clock signal generator 9 generates a channel clock signal 10 shown in FIG. 2(a). In response to the channel clock signal 10, laser driver 11 makes the laser diode 1 emit pulse beam. Laser pulse beam 2 of FIG. 2 is irradiated on the optical disk 8 as a optical spot 4 through an objective lens 3. On the other hand, a magnetic head 5 which is closely disposed to optical disk 8 and is driven by signal generator 6 forms a modulation magnetic field 7 shown in FIG. 2(d). Therefore, a mark of FIG. 2(e), corresponding to a channel bit shown in FIG. 2(b), is recorded on the optical disk 8. As shown in FIGS. 2(a)-2(e), if the frequency of channel clock signal 10 is increased and the laser beam focussed as a spot 4 is irradiated on the optical disk 8 as a pulse, by the combination of the pulse type laser beam and the modulation magnetic field, the optical spot 4 in synchronization with the channel clock signal 10 is irradiated on the optical disk 8. Marks are overlapped and then recorded on the optical disk 8 by the optical spots irradiated like this. According to this method, magnetic pit which has the mark length shorter than the optical spot 4 is recorded. This method is a known technique published in Japanese patent publication Pyungsung No 1-292603.
As a method for retrieving the recorded information from the optical disk, on the other hand, there is a method to focus laser beam with a constant output power as a spot whose diameter is as short as its wavelength and then irradiate as a spot on the surface of the rewritable magneto.sub.-- optical media.
The focussed optical spot is reflected from the surface of the rewritable magneto.sub.-- optical media. At this time, the polarization state of the laser beam is changed by Kerr effect. By optically detecting the change of the polarization state of the reflected beam, the information recorded on the magneto-optical media in magnetic state is read from the media. However, as shown in FIG. 3, as the information is recorded on the rewritable magneto.sub.-- optical media in high density, the length of the magnetic mark is getting shorter and the optical spot is getting longer than the magnetic pit (or mark). As the result, a problem arises in the resolution capability when the mark is read out.
In order to solve this problem, super resolution techniques have been attempted. As one of the techniques, a method of magnetically induced super resolution (MSR) using an exchange coupling force has been introduced. A method of using an in-plane magnetic layer which is a kind of the MSR is shown in FIG. 4. As shown in FIG. 4, the rewritable magneto.sub.-- optical media consists of two layers having an exchange coupling structure between a readout layer with a relatively low coercivity and a recording layer with a relatively high coercivity. The readout layer has the in-plane magnetic layer. However, when the temperature of the layer is over a specific temperature, the readout layer changes its magnetic orientation and has a perpendicular magnetization. The recording layer is a perpendicular magnetization layer so as to preserve the information. If the laser beam is irradiated on the readout layer in order to retrieve the information, the magnetization of the readout layer of high temperature area in the middle of the optical spot (the area with the temperature above threshold value in FIG. 4) is changed from the in-plane magnetization to the perpendicular magnetization, and then a polar Kerr effect comes out. In other words, the magnetic field in the high temperature area of the readout layer is changed into the direction of the magnetic field of the recording layer. On the contrary, because the Kerr effect does not occur in the low temperature area in the neighborhood, the magnetization of the recording layer is masked. Therefore, if the power of the regeneration laser beam is properly selected, the recorded information is retrieved from the high temperature area corresponding to the middle of the laser spot, and, as the result, the retrieving operation in super resolution is possible. However, by the reason that this method of retrieving a small magnetic pit (or mark) by masking the readout layer like this uses a subtle temperature distribution in the beam spot, the change in the magnetic orientation is affected by the fluctuation of the rotation speed of the laser disk and the change in the power of the regeneration laser beam and therefore is unsatisfactory As the result, a good carrier-to-noise ratio is not obtained. As the result, the error rate becomes high and the jitter occurs, and a good quality of the readout signal is not obtained.
As a method to solve this problem, the technique irradiating a regeneration laser beam in a pulse synchronized with the channel clock signal is disclosed in Japanese patent publication Pyungsung 4-325948. According to this technique, there is a merit making the error rate very low.
FIG. 5 shows an example of the apparatus for retrieving the recorded information from the rewritable optical media. Based on the regeneration clock signal of clock generator 58, a pulse shaper 57 outputs a pulse type of signal. In response to this pulse type of signal, laser driver 56 drives the laser diode 55. The laser beam emitted in the pulse type from the laser diode 55 is focussed on the surface of the rewritable optical media 51 by collimator lens 54 and objective lens 52. The laser beam spot focussed on the media 51 is reflected from the readout layer and passed through the objective lens 52, and then comes toward the first polarized beam splitter 53. The optical spot is again applied to the second polarized beam splitter 59 by the first polarized beam splitter 53. In this splitter 59, p-polarization component of the beam spot is transmitted through the splitter 59 and s-polarization component is reflected from the splitter 59.
The p-polarization component and the s-polarization component are focussed and then converted into electrical signals by the first photo detector 61 and the second photo detector 60, respectively. The photoelectric converted electrical signals is applied to the difference amplifier 62. After the signals are amplified, they are applied to the regenerated bit stream detector 63. The regenerated bit stream detector 63 processes the output signal of the difference amplifier 62 and then generates a bit signal corresponding to the recorded information, that is, a binary signal. Usually, the regenerated bit stream detector 63 filters the output pulse signal of the difference amplifier 62 by using a lowpass filter and generates a regenerated bit signal of 0 or 1 by zero.sub.-- crossing the filtered signal. However, this conventional technique has the following weakness. As described above in detail, in the regeneration method irradiating the pulse type of laser beam on the optical disk, the electrical signal detected by the photo pickup is of pulse type. However, in the high density recording media as shown in FIG. 3, the detected electrical signal is too small as compared to size of the pulsed laser spot and is therefore easily corrupted by the noise caused by the laser beam pulse. Therefore, the signal-to-noise ratio is so bad. In FIG. 5, because the s-polarization component and the p-polarization component are very small, the output pulse signal obtained from the difference amplifier 62 is also small. Moreover, the difference between the magnitude of the pulse signal corresponding to high signal 1 and the magnitude of the pulse signal corresponding to low signal 0 is too small. Therefore, it is very difficult to exactly determine 0 or 1 of the regenerated bit signal in the regenerated bit stream detector 63.
As a method to overcome this problem, the method for retrieving the regenerated bit signal without any relationship to the s-polarization and the p-polarization is disclosed in Japanese patent publication So 63-173252. According to this method, as shown in FIGS. 6 and 7, in synchronization with the regenerated clock signal generated at the regeneration clock generator 70, a pulse shaper 71 outputs a pulse signal. Based on this pulse signal, a regeneration beam generator 72 irradiates the pulse type of regeneration beam on the recorded mark of the optical disk 73. At this time, a signal detector 74 detects the electrical signal from the optical disk 73. Sample-and-holder 76 samples and holds the electrical signal in response to a sampling signal. A regenerated bit stream detector 77 obtains the regenerated bit signal from the output signal of the sample-and-holder 76. However, according to the structure in FIG. 6, the time delay from the irradiation of the regeneration beam to the detection of the electrical signal is not considered. Therefore, if the delay time occurs, the detected electrical signal is not sampled on the sampling instant and the right regenerated bit signal is not obtained.